[nj_bartel]

I don't understand the means of the 'divalent' hydrogens in diborane.  I read up on the banana bonding on wiki, but it didn't really clarify it to me.  I understand that the electrons are just delocalized, and there are just the same amount of electrons spread out over a larger system (or at least I think that's how it is), but what causes that, and why isn't it seen in a plethora of other substances?

[Borek]

I have asked over 20 years ago the same question to one of our quantum chemists at Warsaw University (strangely, I think I remember whom I have asked - dr. Bogumil Jezierski). He answered something like "there is no reason, just when you do the calculations this structure has the lowest energy".

[nj_bartel]

How very unsatisfying 

[azmanam]

I'm sure it's not p-chemically accurate, but you can think about it as the B-H sigma bond interacting with the empty p orbital on the other boron.    I've not heard the term banana bond, we always called 3-center-2-electron bonding.  I'm envisioning something similar to an empty d-orbital on a transition metal coordinating with an alkene pi-bond.  Maybe something akin to the first step in the oxymercuration mechanism?

[AWK]

I have asked over 20 years ago the same question to one of our quantum chemists at Warsaw University (strangely, I think I remember whom I have asked - dr. Bogumil Jezierski). He answered something like "there is no reason, just when you do the calculations this structure has the lowest energy".

And calculations proves the real structure seen by X-ray

[Mitch]

I wouldn't have a problem considering strong hydrogen bonds as divalent.

[nj_bartel]

I'm not sure I'd see it that way Mitch - there's hardly any difference in electronegativity in those bonds.

What you said makes it easier to see how it could happen Azmanam.  Does the same type of bonding happen with aluminum hydride?

Edit: Just looked on wiki - yes it does.

[Vidya]

 In diborane there are 12 valency electrons ,three of each boron atom and six from the hydrogen atoms .
Thus the electron deficiency is associated with the bridge groups .They are abnormal bonds as the two bridges involve only one electron from each boron and one electron from each hydrogen atom,making a total of 4 electons .An sp3 boron atom orbital overlap with 1s orbital of hydrogen atom .This  gives a delocalized molecular orbital covering all three nuclei,containing one bridge with one pair of electrons----Three -centre -two -electron bond[/color]